Closed loop operation of induction motor drive systems without shaft transducers is commonly referred to as sensorless, tachless, or transducerless. Eliminating the need for rotor shaft transducers such as encoders, resolvers, and tachometers for position and velocity estimations in an induction motor drive system can reduce system cost, improve system reliability, and reduce the total motor package size.
Several approaches for providing sensorless control of induction motors are described in U.S. Pat. Nos. 5,565,752 and 5,585,709 wherein a determinist spatial modulation in the rotor leakage inductance of the induction motor is tracked via the demodulation of an injected AC signal voltage or current. Tracking of the modulated rotor leakage inductance provides an estimate of the rotor position and velocity that can then be used for sensorless control.
U.S. Pat. No. 5,565,752 describes induction motor rotors with spatial variations in the rotor leakage inductance to enable rotor position and velocity tracking. The patent describes special rotor slot designs that have either open slots or a combination of open and closed slots. Although large motors often have bar wound rotors with open slots, the majority of conventional small induction motors, however, are fabricated with closed rotor slots with the rotor bars being cost-effectively formed by casting molten aluminum (these rotors are usually machined and sometimes the bridges are removed and the slots are opened after casting). U.S. Pat. No. 5,565,752 does not disclose manufacturing methods or rotor designs that are readily manufacturable with existing industrial casting processes and equipment.